Guidelines for group buffer pool thresholds

In most cases, you should use the CLASST and GBPOOLT thresholds to be the corresponding VDWQT and DWQT thresholds for the local buffer pools if these values perform well locally.

Otherwise, you can use the default values (5% for the class threshold and 30% for the group buffer pool threshold). Depending on your workload, these values help reduce disk contention during castout.

If you find that some writes to the group buffer pool cannot occur because of a lack of storage in the group buffer pool, increase the group buffer pool size, or decrease the group buffer pool castout thresholds. One way to tell if this is happening is to see the detail report of the DISPLAY GROUPBUFFERPOOL command. The following figure shows an example report that indicates the lack of storage problem.
Figure 1. Example output of group detail statistics
DSNB783I -DB1A CUMULATIVE GROUP DETAIL STATISTICS SINCE 15:35:23 May 17, 2002
DSNB784I -DB1A GROUP DETAIL STATISTICS
                 READS
                   DATA RETURNED A                    = 3845
DSNB785I -DB1A     DATA NOT RETURNED
                     DIRECTORY ENTRY EXISTED B        = 27
                     DIRECTORY ENTRY CREATED C        = 28336
                     DIRECTORY ENTRY NOT CREATED D    = 332, 0
 
DSNB786I -DB1A   WRITES
                   CHANGED PAGES                      = 20909
                   CLEAN PAGES                        = 0
                   FAILED DUE TO LACK OF STORAGE E    = 8
                 CHANGED PAGES SNAPSHOT VALUE         = 974
DSNB787I -DB1A   RECLAIMS
                   FOR DIRECTORY ENTRIES F            = 18281
                   FOR DATA ENTRIES                   = 47
                 CASTOUTS                             = 16073
DSNB788I -DB1A   CROSS INVALIDATIONS
                   DUE TO DIRECTORY RECLAIMS G        = 4489
                   DUE TO WRITES                      = 3624
                   EXPLICIT                           = 0
DSNB762I -DB1A   DUPLEXING STATISTICS FOR GBP0-SEC
                   WRITES
                     CHANGED PAGES                    = 20909
                     FAILED DUE TO LACK OF STORAGE    = 8
                   CHANGED PAGES SNAPSHOT VALUE       = 974
DSNB790I -DB1A DISPLAY FOR GROUP BUFFER POOL GBP0 IS COMPLETE
DSN9022I -DB1A DSNB1CMD 'DISPLAY GROUPBUFFERPOOL' NORMAL COMPLETION
The field indicated by  E  indicates this type of problem.

Effect of GBPCACHE ALL on guidelines

If you are using a group buffer pool to cache pages as they are read in from disk (GBPCACHE ALL page sets), consider lowering the threshold values to allow more space for caching those clean pages.